The present invention relates to internal combustion engines and more particularly to spark ignited, reciprocating-piston type engines (although other forms of such engines, e.g. rotary engines, are within the broadest scope of the invention as to one or more of its features). The present invention deals with effective utilization of variable air-fuel ratio lean burn modes of operation of internal combustion (IC) engines with preferably electronically controlled fuel injection and ignition timing for improved emissions control, especially control of the more difficult to reduce nitrogen oxide (NOx) emissions, and for improved fuel efficiency. In more optimal form, the invention features the purposeful use of residual gases in the engine in a way that does not reduce low-end torque, and further features, contrary to industry practice, the purposeful use of reverse stratification for lowest NOx emissions. Such approach is made possible by the incorporation of very high power and high energy (VHE) ignition as part of the overall engine design.
The features of the VHE ignition per se, have been disclosed in several prior patents, including U.S. Pat. No. 4,677,960 on voltage doubling, U.S. Pat. No. 4,774,914 on piston firing, U.S. Pat. No. 4,841,925 on enhanced toroidal gap ignition with focussing spark plug tip, on U.S. patent application Ser. No. 07-350,945, now abandoned, and in co-pending U.S. patent application Ser. No. 684,595, now U.S. Pat. No. 5,131,376, entitled "Distributorless Capacitive Discharge Ignition System". These patents and two patent applications are incorporated herein by reference as though set out at length herein.
The present invention can be utilized with a variety of fuels including conventional petroleum-derived hydrocarbon mixture fuels, e.g., gasoline, or non-conventional petroleum and/or plant derived fuels, e.g., methanol, ethanol, natural gas, alcohol-hydrocarbon mixtures, etc. Discussion hereinafter is limited to the conventional fuels. But it will be understood that the points discussed applicable with correction factors well known to those skilled in the art to other fuels.
Some terms used herein are now defined:
(1) Air-Fuel Ratio (AFR):
The weight ratio of air to fuel (usually lbs to lbs or kilograms to kilograms) as the vapor form equivalent of given weights of air and fuel at standard temperature and pressure (STP), all in accordance with standard industry practice which takes AFR of 14.7 to 1 (14.7:1) as a stoichiometric ratio (14.7 lbs of air combusting 1 lb of gasoline, defined also as (AFR)stoich. An alternate expression of AFR is lambda (.lambda.), wherein lambda equals AFR/(AFR)stoich. and hence lambda is 1.0 at stoichiometric mix and STP conditions. The term equivalence ratio phi (.OMEGA.), the inverse of lambda, may also be used. AFR varies in accord with certain engine settings, principally engine speed and manifold absolute pressure (MAP). A carburetted nominally stoichiometric engine typically inducts an AFT of 13:1 at full throttle and ideally 14.7 at part throttle.
(2) Lean-Burn (or Lean of Stoichiometric, or highly dilute):
Operation of an internal combustion engine at AFR above stoichiometric, i.e. in the 16:1 to 18:1 range for current lean burn gasoline engines, but preferably 18:1 to 25:1, a high degree of lean burn, as is contemplated in the present invention, all subject to appropriate adjustment of the range limits for other conditions as indicated above. In terms of lambda, excess air above stoichiometric at AFR of 24:1 is approximately lambda=1.6.
(3) Three way catalyst and Lambda closed loop control:
A multi-mode (three-way catalyst) oxidation-reduction exhaust clean up system which is used in a major portion of the world's automotive engines. A closed loop feedback maintaining lambda close to one is employed in state of the art systems.
(4) Ignition Timing:
The degrees before top dead center (BTDC) of piston stroke (or rotary engine equivalent) where ignition commences. Emissions and specific fuel efficiency as a function of advanced or retarded timing have been substantially studied in the art.
(5) Valve Timing:
The degrees, before and after top dead center (BTDC and ATDC) where the intake valve opens and the exhaust valve closes, and the degrees before and after bottom dead center (BBDC and ABDC) where the exhaust valve opens and the intake valve closes.
(6) EGR-Exhaust Gas Recirculation:
The amount of exhaust or burnt gas (as a percentage) that is recirculated from the exhaust into the unburnt gas, i.e. the fuel-air mixture or charge, which is inducted into the engine cylinders.
(7) Residual Gas or Residual Gas Fraction or Residual:
The amount (fraction) of exhaust gas remaining in the inducted fuel-air mixture after the exhaust valve closes to participate in the combustion of the inducted mixture.
(8) Stratified Charge:
Generally defined to mean the purposeful formation of a non-uniform fuel-air mixture or charge in the engine cylinder prior to combustion, where a locally richer mixture is produced at the spark plug site so as to help ignition of an overall (combustion chamber as a whole) leaner mixture.
(9) Reverse Stratification:
The process (described in this application) whereby a leaner than average combustion chamber fuel-air mixture is formed locally at the spark plug site [or, where the spark plug is located in a region of a premixed fuel-air mixture where the almost homogeneous local mixture is leaner than the average]. A difference of at least one air-fuel ratio is required for the mixture to be considered reverse stratified in the context of the present invention, where the stratification is designated by the two AFR numbers written with a slash between them, the first number being higher for reverse stratification, e.g. 22/20.
(10) Turbulence:
The vigorous agitation of the fuel-air mixture in a combustion chamber, e.g. an engine cylinder characterized by bulk motions (kinetic energy) of the mixture including large scale organized motions such as swirl and squish and small scale disorganized (more random) motions such as microscale turbulence.
(11) Wide Open Throttle or WOT:
The operating condition of an engine in which the throttle or other means controlling air-flow into the cylinder is completely open to permit the maximum amount of air to enter the cylinder.
(12) MAP:
manifold absolute pressure.
(13) MBT:
minimum best timing, the industry-recognized expression for the ignition timing (spark occurence as degrees in advance of top dead center, or "spark advance") that yields best torque (maximum brake torque) for the engine.
Other related terms are defined below as used.
Primary interest in lean burn has come from developers and users of automobile related internal combustion engine technology. These investigators have failed to define ways of realizing the potential benefits of lean burn operation under the full range of conditions of automobile usage, especially with the advent of VHE ignition, to be described presently.
The art of understanding of difficulties with a moderate degree of lean burn (lambda of about 1.1) are expressed in many references, e.g., at page 325 of the 1988 Edition of the Bosch Automotive Electric/Electronic/Systems Manual. See also, pp. 290-291, 415, 417 (motronic map) and 439 of the 1986 Bosch Automotive Manual (2nd Edition) and pp. 81-96 and 348-360 of Arcoumanis, "Internal Combustion Engines" (Academic Press 1988).
It is therefore a principal object of the present invention to provide a range of internal combustion engine apparatus and a range of methods of internal combustion engine operations that realizes, effectively the potential of lean burn, especially in reducing the NOx emissions to the required low levels.